In the prior art, for making this measurement, a manual operation was undertaken. An operator arranged the dried fibers between two glass plates and projected the image thereof on a large screen. The length of the fibers was measured on the projection screen, for example by means of measuring curvimeters used for reading maps or measuring wheels provided with an electronic device such as a photodiode lighted through a wheel provided with thin equidistant slots which accordingly permits to measure the path of the wheel and therefore the length of the fibers on the screen by counting the pulses which are recorded by a data sensor. Those prior art devices are known under the name of semi-automatic sensors. This operation is particularly long and tedious and therefore, in spite of the semi-automatic recording electronic device, only a small number in the range of 50 fibers of a sample was usually measured. Additionally, even in a not conscious way, the operator has a tendency to make a choice or a selection among the measured fibers. In particular, he has a tendency to measure the longest or the nicest elements whereby the consequent statistics are erroneous.
One can easily imagine the time which will be necessary for making a measure on samples corresponding to 50 twenty-year old trees for, for example, a fertilization test with 7 treatments would require 50.times.20.times.7.times.50=350,000 fibers to be measured. Additionally, the effects of the age of the trees, of the orientation in the forest and of the height of the sample, result in a very great number of samples. Accordingly, the operator is tempted to limit the number of measured fibers for each sample and unfortunately chooses very often a number smaller than 50, such a number being not sufficient for permitting the obtention of good statistical measures.